Three element projection lens

ABSTRACT

A projection lens comprising three air spaced elements, namely a positive meniscus element, a negative biconcave middle element and a positive biconvex rear element, with each element being formed of a glass having an index of refraction in excess of 1.90 or an Abbe number less than 40.

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Unlted Sta Q6 1111 3,838,910

I Ruben Oct. 1, 1974 [5 THREE ELEMENT PROJECTION LENS 2,720,816 10/1955 Sandbach 350/226 Inventor: Paul Lewis Ruben, Penfield N-Y. 3,160,700 12/1964 Snyder 350/226 Assign: Eastman Kodak p y Primary ExaminerRonald L. Wibert Rochester Assistant Examiner--Conrad Clark 22 i Apr. 24 1973 Attorney, Agent, or FirmDonald D. Schaper [21] App]. No.: 354,046

[57] ABSTRACT 52 us. c1. 350/176, 350/226 A Projection lens Comprising three Spaced [51] Int. Cl. G02b 1/00 mems namely a positive meniscus element a negative 158 Field 61 Search 350/2 176,226 biconcave middle element and a Positive biconvex rear element, with each element being formed of a l 5 References Cited v glass having an index of refraction in excess of 1.90 or UNITED STATES PATENTS an Abbe number less than 40.

2,270,234 l/l942 Warmisham 350/176 6 Claims, 1 Drawing Figure THREE ELEMENT PROJECTION LENS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to lenses and in particular to a three element lens which is adapted for the projection of images from microfilm.

2. Description of the Prior Art The quality required of a projection lens is partially dependent upon its intended use and inversely dependent on the image magnification factor. For example, projection of micro-images usually requires less magnification than does ordinary 35mm. slide projection, so that lenses having better resolution and aberration corrections are usually used for microfilm projection readers. Still greater lens quality may be requied for projection printing magnified copies from microflim, since the magnification factor is commonly smaller for such copiers than for most microfilm projection readers. Known lenses of suitable quality for use as micro-image projectors have generally consisted for four or more elements. Such lenses are usually more expensive than a three element lens because of the additional cost of manufacturing and mounting an additional element.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide three element triplet lenses suitable for use in projection systems and in particular for use in the projection of images from microfilm.

It is another object of this invention to provide such triplet lenses which are well corrected for distortion, spherical, comatic, astigmatic and chromatic aberrations.

It is a still further object of this invention to provide such triplet lenses which are characterized by a curved image field.

It is also an object of this invention to provide such triplet lenses in which the elements are characterized by indices of refraction in excess of 1.90.

These and other objects are accomplished according to the present invention by triplet lenses comprising three air-spaced elements manufactured from high index glasses and arranged from front to rear in the following order: a front positive meniscus element; a middle biconcave negative element; and a rear positive biconvex element.

BRIEF DESCRIPTION OF THE DRAWING In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawing which is a diagrammatic cross-section of a lens according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS of the spectrum. The Abbe numbers are calculated utilizing the D-line index and, as the main dispersion, the index difference between the 0.4861 micron hydrogen F-line and the 0.6563 micron hydrogen C-line. Radii of curvature having centers of curvature to the right of the surface are considered positive; those with centers of curvature to the left of the surface are considered negative. All parameters are based upon a lens focal length of millimeters.

In all embodiments of the invention as illustrated in the drawing, the lens comprises three air-spaced elements. Element 1 is a front positive meniscus element. Element 2 is a middle negative biconcave element. Element 3 is a rear positive biconvex element.

Lenses may be made according to this invention by following the specifications in the preferred embodiments presented below:

EXAMPLE I Thickness or Element Radius (mm) Separation (mm) EXAMPLE II Thickness or Element Radius (mm) Separation (mm) N V The lens described in Example II is designed to project am image which is magnified by a factor of 37.905 onto a screen which is concave to the lens and which has a spherical surface with a inch radius. This lens is further characterized by a relative aperture of f/2.50 and a half-field angle of 20.00.

EXAMPLE III Element Radius (mm) fl 2.80 Thickness or Separation (mm) N V EXAMPLE lll-Continued focal length utilized in Examples l-lV. The three and four element lenses are characterized by the focal leng- F .L.=lmm Thickness m 02.80 th and relative aperture listed for each lens. Element Radius (mm) Separation (mm) N v Table 1 lists modulation transfer function (MTF) 51:9'45 data for two different spatial frequencies, with both R,=-225.71 saggital and tangential performance at differing field 2 R5389 "94910 positions. The MTF is the efficiency of transfer of the S,=8.89 amplitude of spatial waves through a lens. The MTF for 3 R*=98"549 TH? 194283 an ideal lens is limited by diffraction, in the absence of R,,=-98.549 aberrations. The presence of aberrations will tend to limit the MTF further. Thus, a lens which is characterized by a higher MTF than a second lens is generally p gj zi ig gfsg zsfi gl g if g 1 95;: g; 1 characterized by reduced aberrations. In addition, a 33.232 onto a screen which is concave to the lens and 5 Smaller MTF Fange for filfferem field-posmcmS mdlhas a spherical surface with a 144.5 inch radius. The Gates umform quahty across the Image f By lens is further characterized by a relative aperture of Comparing the MTF data for Examples with the f/zso and a balm-131d angle of 205 data for the three and four element lenses, it may be seen that applicant s lenses are generally characterized EXAMPLE W by reduced aberrations and more uniform image quality, particularly when compared with a lens having the flFloomm Thickness or same focal length and relative aperture.

Element Radius (mm) Separation (mm) N v Particular attention should be paid to the index of re- RF 8 fraction of the glasses from which the lens elements are l T=10 597 1494283 361 made. Each of the indices is extremely high, all being R2=l52-l5 S :10 120 in excess of 1.90. The use of these high index glasses aids in achieving the aberration corrections which re- 2 R 567 2= 194910 sult in the improved optical performance illustrated in 52:8982 0 Table 1. In addition, element 3 is symmetrical in de- R,=97.978 sign, which, while not required to achieve improved op- 3 Rsbgwm TFIG'HO "94283 3 tical performance, reduces the cost of construction of the lens by eliminating the possibility of reversal of this element during assembly.

The lens described in Example IV is designed to Although the invention has been described in detail project an image which is magnified by a factor of with particular reference to certain preferred embodi- 29.031 onto a screen which is concave to the lens and ments thereof, it will be understood that variations and has a spherical surface with a 127.2 inch radius. The modifications can be effected within the spirit and lens is further characterized by a relative aperture of scope of h invention,

f/2.80 and a half-field angle of 20.50". I l i The optical performance of lenses constructed in acl, A three element lens comprising, from front to cordance with the parameters listed in Examples l-lV rear, a positive meniscus element, a biconcave negative is illustrated in Table l, with a comparison to presently element and a biconvex positive element, each of said available three and four element lenses utilizing meelements being formed of aglass having an index of redium index glasses. The lenses of Examples l-lV were fraction in excess of 1.90, wherein the lens has a focal constructed with the focal length and relative aperture length F and the radii of curvature, R, the thicknesses, listed under each example, rather than with the 100mm T, and the air spaces, S, as numbered by subscript from TABLE 1 Modulation Transfer Function Spa- Example 1 Example 2 Example 3 Example 4 3 Element 4 Element 4 Element gal 10mm f12.5 10mm f[2.5 14mm 1728 16mm 172.8 19mm fl2 5 26mm 1725 1 quencyField Sag. Tan. Sag. Tan. Sag. Tan. Sag. Tan. Sag. Tan. Sag Tan. Sag. Tan

lines Posi lmm tion front to rear, are as defined by the following inequalities:

2. A three element projection lens comprising, from front to rear, a positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:

Thickness or Element Radius (mm) Separation (mm) N V wherein from front to rear, the lens elements are numbered from 1 to 3, the corresponding indices of refraction, N, and Abbe numbers, V, are listed for each of the lens elements, the radii are numbered from R, to R,, the thicknesses are numbered from T, to T, and the air spaces are numbered from S, to S 3. A three element projection lens comprising, from front to rear, a positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:

Thickness or Element Radius (mm) Separation (mm) N V R,=49.53 I T,=l7.06 l.94283 36.1

S,=7.90 R,=-l90.47 2 T,=8.00 1.93664 20.8

S,=7.33 R,=94.442 3 T,=l3.62 1.94283 36.!

focal length of millimeters when constructed according to the parameters in the following table:

Thickness or wherein, from front to rear, the lens elements are numbered from 1 to 3, the corresponding indices of refraction, N, and Abbe numbers, V, are listed for each of the lens elements, the radii are numbered from R, to R,, the thicknesses are numbered from T, to T and the air spaces are numbered from S, to 8,.

5. A three element projection lens comprising, from front to rear, 21 positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:

Thickness or wherein, from front to rear, the lens elements are numbered from 1 to 3, the corresponding indices of refraction, N, and Abbe numbers, V, are listed for each of the lens elements, the radii are numbered from R, to R,, the thicknesses are numbered from T, to T and the air spaces are numbered from S, to S,

6. A three element lens comprising, from front to rear, a positive meniscus element, a biconcave negative element and a biconvex positive element, each of said elements being formed of a glass having a Abbe number which is less than 40, wherein the lens has a focal length F and the radii of curvature, R, the thicknesses, T, and the air spaces, S, as numbered by subscript fron front to rear, are as defined by the following inequalities:

* I t i i P0405) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,838,910 Dated Octohgr; 1-. 197g Inventofls) PaulvL. Ruben I I It: is certified that erfor appears in the above-identified patent; and that said Letters Patent are hereby corrected as shown below:

Columns 3 and 4, (Table I), line 51, after '"119mm" delete "f/2-'.5" and insert --f/3.5- T

Signed ahdasealed this 17th da of December 1974.

(SEAL) Attest: I

McCOJ M. GIBSON JR. c. MARSHALL DANN- Attesting Officer .7 Commissioner of Patents, 

1. ,A three element lens comprising, from front to rear, a positive meniscus element, a biconcave negative element and a biconvex positive element, each of said elements being formed of a glass having an index of refraction in excess of 1.90, wherein the lens has a focal length F and the radii of curvature, R, the thicknesses, T, and the air spaces, S, as numbered by subscript from front to rear, are as defined by the following inequalities: 0.40F<R1<0.50F1.5F<R2<2.0F1.8F<-R3<2.4F0.35F<R4<0.45F 0.90F<R5<1.0F 0.90F<-R6<1.0F 0.10F<T1<0.20F 0.05F<T2<0.10F 0.10F<T3<0.20F 0.05F<S1<0.15F 0.05F<S2<0.10F
 2. A three element projection lens comprising, from front to rear, a positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:
 3. A three element projection lens comprising, from front to rear, a positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:
 4. A three element projection lens comprising, from front to rear, a positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:
 5. A three element projection lens comprising, from front to rear, a positive meniscus element, a biconcave element and a biconvex element, said lens having a focal length of 100 millimeters when constructed according to the parameters in the following table:
 6. A three element lens comprising, from front to rear, a positive meniscus element, a biconcave negative element and a biconvex positive element, each of said elements being formed of a glass having a Abbe number which is less than 40, wherein the lens has a focal length F and the radii of curvature, R, the thicknesses, T, and the air spaces, S, as numbered by subscript fron front to rear, are as defined by the following inequalities: 0.40F<R1<0.50F 1.5F<R2<2.0F 1.8F<-R3<2.4F 0.35F<R4<0.45F 0.90F<R5<1.0F 0.90F<-R6<1.0F 0.10F<T1<0.20F 0.05F<T2<0.10F 0.10F<T3<0.20F 0.05F<S1<0.15F 0.05F<S2<0.10F 